Among all digital cellular mobile communication systems, the GSM communication system is the most developed and has the largest market occupation. It employs the digital wireless communication technology in which FDMA and TDMA are combined.
One of the effective measures for improving the system capacity of GSM communication system is to increase the utilization factor of the spectrum, specifically, is to decrease interference between channels. At present, there are three kinds of main methods for decreasing the interference: automatic power control, frequency hopping and discontinuous transmitting (DTX). Here, DTX is essentially a kind of power control method. Therefore, the power control in GSM is an important means for decreasing the multiple access interference. It not only can significantly improve the channel quality, but also can increase the system capacity to a large extent.
The principle of the automatic power control is that the transmitting power of a transmitter will be adaptively decreased under a condition that the excellent transmission quality can be obtained without the maximum transmitting power, namely, the even transmitting power of mobile stations and base stations will be reduced if it can be ensured that transmitting quality is greater than a given threshold, thereby the interference to other channels can be decreased. The power control occupies an important position in transmitting technology, specifically, the performance quality of a transmitter mainly depends on the performance of the power control. In practice, multiple terms in the wireless specification of GSM aim at the power control.
As shown in FIG. 1, a traditional narrow band GSM base station normally employs a close-loop power control method that performs single power control for each carrier. The control procedure is as follows. A positive direction power detection voltage outputted from a power amplifier is detected by a detector 105, then a subtraction is implemented in an adder 104 for the above-mentioned voltage and a template curve 106 obtained in advance according to a certain algorithm, the resultant difference is used to control an attenuator 101 in transmitting passage through an integrator 103, thereby the transmitting power is modified. If the power voltage detected at a certain time is lower than a known template voltage 106, the voltage outputted by the integrator 103 will be increased, the attenuation of the attenuator 101 will be decreased, and the output power will be increased to facilitate the increasing of the positive direction power detection voltage. In contrast, if the detected voltage is higher than the template voltage 106, the voltage outputted by the integrator 103 will be decreased, the attenuation of the attenuator 101 will be increased, and the final output power of the power amplifier will be decreased, thereby the voltage outputted by the detector will be decreased.
The power control of a broadband multi-carrier GSM system is a new subject occurring in an evolvement procedure of GSM base station from narrow band to broadband, and it is also a problem that must be solved in developing the broadband multi-carrier GSM technology. Since there is a large difference between the structure of a broadband base station and that of a traditional base station, it is difficult to apply the traditional single-carrier power control method to a broadband base station. So far, the information on the multi-carrier power control of GSM has not been found.
A multi-carrier signal is a combination of the respective single-carrier signals. Because the frequency points and powers of the respective carriers are variable and the envelope of a multi-carrier combination signal is difficult to be predicted, it is difficult to detect the power envelopes of the respective carrier frequencies with a simple and easy method. While the power envelope of each carrier can be detected in real time is a premise condition to implement the close-loop power control, therefore, a great difficult exists in implementing the close-loop power control for multi-carrier signals.